1. Field of the Invention
The present invention relates to a subscriber's line switching control system in which, in a digital switching system accommodating a subscriber's line, an efficient switching control can be carried out for both a digital subscriber's line and an analog subscriber's line.
2. Description of the Related Art
An analog subscriber's line is usually used as a subscriber's line accommodated in a digital switching system, but because of developments in digital techniques, a digital subscriber's line is also used therein, as disclosed in U.S. Pat. No. 4,571,721. In this disclosure, the analog subscriber's line and the digital subscriber's line are combined, and thus a satisfactory interface is required.
In a remote line concentrator having a construction in which the digital subscriber's line and the analog subscriber's line are accommodated, a concentrator stage of a part of a master office is set at a remote location, and a connection control is carried out in accordance with switching connection control information from the master office, to process the digital signal in the same way as in a digital switching system.
The interface between the digital subscriber and the switching system is prescribed by the CCITT (International Telegraph and Telephone Consultative Committee), and therein it is specified that the call control information which controls the low speed digital data and the call information, etc., via the digital subscriber's line from the digital subscriber is sent on the D channel (16 kb/s), and a high speed data including voice information, etc., is sent on the B channel (64 kb/s). In the subscriber's line, the D channel and the B channel are separated, and the data in the D channel is transferred to a D channel handler.
In such a remote line concentrator, the call control information for analog subscribers is received via a scan/signal separator, and the call control information for digital subscribers is received via the D channel handler. Therefore, there is a drawback in that the construction becomes complex because the interfaces of the call control information are different for analog subscribers and digital subscribers.
Further, the D channel handler for digital subscribers and the scan/signal separator for analog subscribers are centralized, which requires comparatively large scale circuitry to provide capacity for many of each type of subscriber. Also, to allow the ratio between digital subscribers and analog subscribers to change the total capacity of the D channel handler and the scan/signal distributor must be greater than the total number of subscribers which is uneconomical.